AB-INITIO MOLECULAR-ORBITAL STUDY OF THE MECHANISM OF PHOTODISSOCIATION OF TRANS-AZOMETHANE

The mechanism of photodecomposition of trans-azomethane (CH3-N=N-CH3-- >2CH(3) .+N-2 has been investigated with high level ab initio molecula r orbital calculations. Potential surfaces of the low-lying electronic states were explored by state-average complete active space self-cons istent-held (sa-CASSCF) and multireference configuration interaction w ith single and double excitation (MRCISD) methods. The calculated vert ical excitation energies for S-0-->S-1 and S-0-->T-1 transitions are i n good agreement with experiments. The lowest crossing point between t he S-0 and S-1 surfaces, around which excited molecules would make eff icient internal conversion to the ground state, is found to be asymmet rical with a CNNC dihedral angle of 92.8 degrees and two CNN angles of 132.0 degrees and 115.6 degrees, respectively. Transition structures for both simultaneous and sequential C-N bond cleavages on the S-0 sur face were found. Though the activation energy of sequential C-N bond c leavage is about 7 kcal/mol higher than that of the simultaneous C-N b ond cleavage, the Gibbs free energy of activation is lower above 0 deg rees C, indicating that thermal decomposition of trans-azomethane is s equential. Photodissociation is expected to take place sequentially as well. In the sequential mechanism, dissociation of the first C-N bond on the S-0 surface takes place endoergically without reverse barrier resulting in CH3N2 intermediate, which should decompose almost immedia tely over a barrier of less than 1 kcal/mol. Thus, the photodissociati on reaction is highly asynchronous but is nearly concerted. This mecha nism can explain two seemingly contradictory photodissociation experim ents that two methyl radicals have very different translational as wel l as internal energies and that the velocity vectors of the three frag ments are strongly correlated. (C) 1996 American Institute of Physics.